This invention relates in general to transponder apparatus and more particularly to an improved reply radar transponder adaptable for use within a radar target, such as a missile or other airborne craft, and wherein delayed reply pulses may be generated and transmitted in response to received interrogation pulses from ground or tracking radars. It is intended that such delayed reply pulses be offset in frequency by a predetermined, precise amount, but nevertheless, be phase coherent with respect to the received incoming interrogation pulses.
Radar transponders of the type herein disclosed are adapted for inclusion within a vehicle acting as a moving radar target so as to provide appropriate reply pulses in response to incoming interrogation pulses from a tracking or ground station radar. The power level of the reply pulses is at substantially higher power levels than would be otherwise obtained from echos or "skin tracking" pulses reflected by the transponder-bearing craft or target. In this manner, the tracking range of the radar system as a whole is substantially extended.
In systems of this type, it is known to effect a delay in the transmission of the associated reply pulses for some predetermined time so as to avoid interference with "skin echos" reflected from the surface or surfaces of the transponder bearing vehicle. In addition, it is also known that the reply pulses may be generated and transmitted in phase coherence with the received interrogation pulses so as to preserve the appropriate Doppler frequency information used to determine instantaneous velocity of the referenced target. By phase coherence, it is meant that there is some predictable (in the sense of, "known to the radar") phase relation that is established and maintained between the received interrogation pulses and the transmitted, albeit delayed, reply pulses. Notwithstanding the foregoing, interferences still may be encountered in the prior systems, notably from ground returns and/or other passing reflections. This is possible because interrogations pulses and the associated reply pulses have heretofor been maintained at substantially the same carrier frequency.
Accordingly, it is an object of the present invention to provide an improved radar reply transponder for use in existing radar systems which effectively eliminates interference from ground returns and other undesirable passive reflections.
Another object of the present invention is to provide an improved radar reply transponder for precision pulse Doppler velocity measuring radar systems wherein long term frequency stability for the transponder's local oscillator as heretofore required may be substantially relaxed.
A more particular object of the present invention is to provide improved reply transponder apparatus which provides an appropriate reply pulse in response to interrogation from the tracking station, which reply pulse is phase coherent but delayed in time and offset in frequency.
Still another object of the present invention is to provide an improved reply transponder of the foregoing type wherein the instantaneous frequency of each transmitted reply pulse is offset in frequency from that of the received interrogation pulse by a predetermined, selectable, but precise amount.
Yet another object of the present invention is to provide an improved reply transponder of the foregoing type wherein the down-converting of received interrogation pulse information and the up-converting of the offset reply pulse to be transmitted is effected by a common local oscillator with a switchable reactive element.
A further object of the present invention is to provide an improved transponder of the foregoing type wherein the down-converting of received interrogation pulse information and the up-converting of the offset reply pulse to be transmitted is effected by a common frequency modulated local oscillator.
A still further object of the present invention is to provide an improved reply transponder of the foregoing type wherein the down-converting of a received interrogation pulse information and the up-converting of the offset reply pulse to be transmitted is effected by a common moduated oscillator which is driven by an input signal derived by selecting a harmonic of the pulse rate frequency.